Attacks on web sites in recent years has resulted in severe disruption in network services. These attacks can take any one of a number of forms including, but not limited to, SYN flooding.
In a SYN flooding attack an attacker overloads a victim's site to the point where it cannot cope with incoming traffic. Such an attack, typically, focuses on an inherent characteristic of TCP based services.
Essentially, TCP services rely on a three-way hand shaking protocol on connection set up. A client wishing to make connection with a host sends a synchronization signal (SYN) to the host and the host responds to the client with a SYN acknowledgement (ACK) reply. The client then returns an acknowledgement and the connection is established. The handshaking protocol is illustrated in FIG. 1.
Upon completion of a connection the client forwards a finish (FIN) packet to the host indicating that there will be no further data or packets directed to the host and the connection is thereafter closed.
In a SYN flooding attack the attacker will typically use a false or invalid source address such that when the host returns the SYN/ACK message it does not reach a valid client. Under the TCP protocol the host stores half opened connections i.e. connections for which the third leg of the three way protocol has not been completed for a set period of time or until a system time out occurs. If, during this time interval multiple new half opened connections are established at the host site the memory allocated to retaining such connections becomes swamped and eventually is unable to receive any more SYN packets. At this stage the server or host will crash or will not respond to any new connections and the site goes out of service. Because the host is unable to receive further data the attacker has been successful in generating what is known as a denial of service attack. Denial of service attacks have become an increasingly prevalent form of a security threat and the problem, so far, has been quite difficult to solve. Several countermeasures have been proposed and can be characterized as firewall and router filtering, operating system improvements, protocol improvements and intrusion detection.
Considerable prior art exists in the area of security attacks and the problem is well described in a publication by C. Schuba, I. Krsul, M. Kuhn, E. Spafford, A. Sundaram and D. Zamboni entitled “Analsyis of a denial of service attack on TCP”, published in the Proc., 1997 IEEE Symp. Security and Privacy. The Schuba et al. paper describes the problem and the classical solutions for a proxy service: the TCP relay and the semitransparent TCP gateway. In those two solutions, a fire wall intercepts the TCP connections, maintains the states of the TCP state machine and introduces new packets to avoid the attacks.
The proxy solutions according to this publication have to maintain, for each connection, the states of the corresponding TCP state machine. This mechanism needs a lot of resources and can be itself the target of a new denial of service attack. Thus, the high cost of the computation overhead makes this solution inappropriate for network routers or switches.
A second prior art solution which is closer to the present invention is described by H. Wang, D. Zhang and K. G. Shin, “Detecting SYN flooding attacks”, Proc. Infocom 2002. The method according to Wang et al. relies on a counting argument on the SYN and FIN packets on the TCP connections. Those packets should go in pairs in any well behaved connection. Thus, the number of SYN packets should match roughly the number of FIN packets. The simplicity of this method lies in the stateless and low computation overhead which makes the detection mechanism itself immune to flooding attacks. This simplicity allows the detection to be performed in the leaf routers that connect end hosts to the Internet.
The counter mechanism according to Wang et al. has a major drawback. If attackers know exactly the detection protocol described by this method they may thwart this approach simply by overflooding the routers with synchronized SYN and unrelated, over even invalid FIN packets. Hence the SYN counter and the FIN counter would be roughly the same. However, the victim's TCP/IP stack would be open to many half-opened connections that would not be closed by the invalid FIN packets. This corresponds to the original denial of service attack.
U.S. Pat. No. 6,321,338 which issued Nov. 20, 2001 to Porras et al. and entitled “Network Surveillance”, also provides prior art for this technology. According to the Porras et al. patent there is provided a method of network surveillance including receiving network packets handled by a network entity and building at least one long term and at least one short term statistical profile from a measure of the network packets that monitors data transfers, errors or network connections. A comparison of the statistical profiles is used to determine whether the difference between the statistical profiles indicates suspicious network activity.
This patent further discloses, in addition to the details mentioned above, that intensity measures of event streams e.g. ICMP packets, are particularly suited for detecting flooding attacks. Furthermore, the patent discloses that intensity measures that correlate SYN to SYN _ACK messages, volume analysis of SYN/RST messages or TCP/FIN messages are useful to detect port availability or scanning attacks.